Support for using finder semantic config with index_name

In the docs, it says that you can use a finder for the [entire index] (https://github.com/FriendsOfSymfony/FOSElasticaBundle/blob/master/Resources/doc/usage.md#searching-the-entire-index) and that you can use [index_name](https://github.com/FriendsOfSymfony/FOSElasticaBundle/blob/master/Resources/doc/setup.md#c-basic-bundle-configuration) to refer to the index via a different name.

However, these two options are currently mutually exclusive, as there is no ElasticaToModelTransformer available for a finder defined on the aliased index service.

ex:
```yaml
fos_elastica:
    clients:
        default: { host: localhost, port: 9200 }
    indexes:

        # Index aliases
        content:
            client: default
            index_name: content_mywebsite
            finder: ~

        # Index definitions
        content_mywebsite:
            client: default
            finder: ~
            types:
            # ...
```

This PR adds support for using a finder on an aliased index service by adding an $alias parameter to loadIndexFinder() and adjusting the parameters if using the index_name semantic config option.